There is such a conventional known assembly in which an instrument panel made of a synthetic resin and an air-conditioning duct made of a synthetic resin are vibration-welded to each other. In this case, the instrument panel is comprised of a back-side molded panel member formed by an injection molding, and a skin affixed to a surface of the back-side molded panel member through an adhesive. The air-conditioning duct is formed by a blow molding, so that it has a plurality of branched-off duct portions, and pinched-off portions connected to the duct portions are used as portions to be welded.
However, the conventional instrument panel suffers from the following problems: An operation for applying an adhesive to the back-side molded panel member and a foam-molding operation for affixing the skin are requisite, and time for curing the adhesive is also required, resulting in an increased number of steps for producing the instrument panel and thus the assembly.
In the back-side molded panel member, the following problem also is encountered: If a thickened portion such as a member-mounting protrusion is formed on a back of the back-side molded panel member, a sink mark corresponding to the thickened portion, namely, a recess is formed on a surface of the back-side molded panel member, and the skin is conformed to the recess. For this reason, the uniformity of the surface of the instrument panel is reduced, resulting in a reduction in commercial value of the assembly.
A further problem encountered in the conventional assembly is that the weight of the assembly is increased as a result of the air-conditioning duct including the portions to be welded, which are extra from the viewpoint of its intrinsic function.